Blender always adds a .blend extension to my image, and can't be opened
.png and .jpg (or .jpeg) are formats for 2D images, not for blender projects.

To save the 3D project use Ctl S or File Save or Save As



Doing this you are saving the whole project: 3d models, all of the 3D environment and the work session (cameras, modifiers, meshes, animation, screens, etc). All of that information gets stored using blender's unique format that uses a .blend extension, that can only be opened in blender.

Note that Rendered images will not be saved within the blender file, unless they are saved as external files and packed into the .blendd Unsaved/unpacked images will otherwise be lost for ever once the file is closed.

If you want to export an image using .png or .jpg (or any of the supported image formats) you have to render your scene into an image first.  Use F12, and then save it using F3 (or save as image).



Then you'll be presented with an option for the format you want to save your image in.




A png or jpg file will be just a rendered image, IT WILL NOT CONTAIN THE 3D MODEL, NOR YOUR BLENDER SESSION.

For more info please read:

https://www.blender.org/manual/data_system/files/save.html?highlight=save

and

https://www.blender.org/manual/render/output/display.html?highlight=save%20image